It is well known that a foamed article produced by expanding foamable polystyrene resin particles obtained by impregnating polystyrene resin particles with a blowing agent is broken easily. To improve such a defect, there has been proposed a process for mechanically blending a styrene-butadiene block copolymer with a polystyrene resin (cf. JP-B-47-17465 and JP-A-54-158467) and a process employing particles of a resin which is excellent in impact strength and so-called high impact polystyrene resin (HIPS), wherein the particles are prepared by polymerizing a styrene monomer in the presence of conjugated diene polymer particles (JP-B-47-18428 and JP-A-7-90105).
However it is necessary for those foamable polystyrene resin particles to be subjected to pelletizing through extrusion molding, which results in increase in cost. Further in commercially available high impact polystyrene resin, a molecular weight of its polystyrene cannot increase beyond a certain level.
JP-A-6-49263 discloses foamable polystyrene resin particles prepared by impregnating polystyrene resin particles with a mixture obtained by adding a polymerization initiator to a monomer mixture of a styrene monomer and conjugated diene monomer, polymerizing the monomer mixture to give polystyrene resin particles containing the obtained copolymer rubber particles gathering densely around the surface portion of the resin particle, and then impregnating the polystyrene resin particles with a blowing agent. Also it is disclosed that the center portion of the foamable resin particle is rich with polystyrene and, since the blowing agent is kept in the center portion, the resin particles having high expansion ratio can be obtained. Further, it is described that the resin before impregnated with the blowing agent has impact resistance.
The process mentioned above does not require a pelletizing step and is excellent from the viewpoint of cost. However the obtained foamed article, particularly the foamed article produced by molding pre-expanded particles within a mold is low in percentage of fused particles (fusion rate) and is insufficient in break resistance (in impact strength by a falling weight method).
An object of the present invention is to provide foamable modified polystyrene resin particles which can be prepared without being subjected to pelletizing and give a foamed article having excellent break resistance and high fusion rate; a process for preparation thereof; and a foamed article produced by expansion-molding the foamable modified polystyrene resin particles.